The Mammalian Heart

Mammals are complex multicellular organisms made up of numerous cells and tissues. For efficient execution of life processes, simple diffusion is not sufficient. Therefore, mammals have an advanced transport system consisting of the heart, blood, and blood vessels.

The mammalian heart

A key example of a mammalian heart is the human heart. The human heart is about the size of a clenched fist and is located in the chest cavity, between the two lungs.

External structure of the mammalian heart

The heart is broader at the top and narrower at the bottom, and is encased in a double layer of tough, inelastic membranes known as the pericardium. These membranes prevent over-expansion during rapid heartbeats and secrete pericardial fluid to allow smooth movement between the layers.

External structure of the mammalian hear

The heart wall is composed of cardiac muscle, which never tires and works continuously as long as the individual is alive. This muscle type is found exclusively in the heart.

The heart wall consists of three layers:

1. Epicardium – The outer protective layer.
2. Myocardium – The middle, muscular layer.
3. Endocardium – The innermost layer, which is continuous with the blood vessel lining.

The coronary artery supplies the heart with oxygenated blood, while the coronary vein removes waste products. The vena cava and pulmonary veins carry blood from the body to the heart, and the aorta and pulmonary artery transport blood from the heart to other parts of the body.

Internal structure of the mammalian heart

The heart has four chambers: the right auricle, right ventricle, left auricle, and left ventricle. The auricles (plural of atrium) have thinner walls compared to the ventricles because they only pump blood to the ventricles, while the ventricles pump blood throughout the body. The left ventricle has thicker walls than the right because it pumps blood to the entire body, while the right ventricle only pumps blood to the lungs.

Internal structure of the mammalian heart

The heart has several valves that ensure blood flows in only one direction:

• The tricuspid valve is between the right auricle and right ventricle.
• The bicuspid valve is between the left auricle and left ventricle.
• Semilunar valves are at the bases of the pulmonary artery and aorta to prevent blood from flowing back into the ventricles.

The septum, a thick muscular wall, divides the left and right sides of the heart, preventing the mixing of oxygenated and deoxygenated blood.

Blood flow through the heart

1. The vena cava brings deoxygenated blood to the heart.
   • Superior vena cava transports deoxygenated blood from the upper body (head, neck, and limbs).
   • Inferior vena cava transports deoxygenated blood from the lower body (limbs, kidney, liver, etc.).
2. Blood enters the right auricle, where it is pumped into the right ventricle via the tricuspid valve.
3. The right ventricle contracts, pushing blood into the pulmonary artery, which transports it to the lungs for oxygenation.
4. Oxygenated blood from the lungs enters the left auricle through the pulmonary vein.
5. The left auricle contracts, pumping blood into the left ventricle via the bicuspid valve.
6. The left ventricle then contracts, pushing oxygenated blood into the aorta, which branches into arteries that carry blood throughout the body.

The heart beats in a coordinated fashion, where the auricles contract while the ventricles relax and vice versa. The sinoatrial node (SAN), located in the right atrium, acts as a pacemaker, regulating the timing and rate of the heart's contractions.

Adaptations of the heart to its functions

Adaptation	Function
Muscular Walls	Contract to pump blood
Cardiac Muscle	Contracts and relaxes continuously without fatigue, ensuring continuous blood flow
Valves	Ensure one-way blood flow
Septum	Separates oxygenated and deoxygenated blood
Connection to Blood Vessels	Enables the transportation of blood to and from the heart
Sinoatrial Node	Sets the time and rate of cardiac muscle contractions
Coronary Artery and Vein	Supply oxygen to and remove waste from the heart